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  <a href="http://sudopedia.enjoysudoku.com/Uniqueness_Test.html#Hidden_Unique_Rectangle">Hidden Unique Rectangle</a>
   <p>
   If <b>digit {5}</b> were entered into <b>cell {3}</b> then:<br>
   <ol>
   <li><b>cell {1}</b> would have to contain <b>digit {4}</b> because there's no other cell in <b>{6}</b> for this value</li>
   <li><b>cell {2}</b> would have to contain <b>digit {4}</b> because there's no other cell in <b>{7}</b> for this value</li>
   </ol>
   Hence the four <b>cells {0}, {1}, {2} and {3}</b> would form a <a href="http://sudopedia.enjoysudoku.com/Deadly_Pattern.html">deadly pattern</a> because there were two possibilities to
   place the two <b>digits {4} and {5}</b> into the four cells.<br/> 
   This would lead to an invalid puzzle with two solutions (a valid puzzle can have just one solution).<br/>
   To avoid the deadly pattern we have to discard <b>digit {5}</b> as a candidate from <b>cell {3}</b>. 
   </p>
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